1. Field of the Invention
The present invention relates to a cathode ray tube used, for example, in a video display device, such as a television, and to an electron gun used in the cathode ray tube.
2. Description of the Prior Art
With the advent of high definition television (HDTV), a need exists for a display device that is able to resolve a resolution of, for example a 1080i (e.g., 1920 Horizontal pixels.times.1080 Vertical pixels) image. Cathode ray tubes (CRTs) of a direct view televisions employ a mask that makes it difficult for such a display to resolve the full resolution of a high definition signal while maintaining a sufficient light output. Further, the desire for large screen display devices have increased over the years. Accordingly, projection-type display devices, such as, for example, rear projection televisions (RPTV) and front projection televisions (FPTV) have increased in popularity.
Rear projection televisions and front projection televisions generally employ a plurality of CRTs (e.g., a red CRT, a green CRT, and a blue CRT) to project a color image onto a screen. Such televisions generally employ CRTs having a face diameter of between 7 inches and 12 inches. It is noted that in general, the larger the diameter of the CRT face, the higher the resolution capability of the CRT. A lens is generally mounted to the front of each CRT to magnify the projected image to a desired size.
Two methods are known for focusing the image produced by an electron beam of the CRT; namely, an electrostatic focusing system and a magnetic (electromagnetic) focusing system. In order to achieve a high resolution display, it is necessary to reduce a spherical aberration in an electrostatic lens or a magnetic lens. The spherical aberration can generally be reduced by increasing the lens diameter.
The outside diameter of an electrostatic lens is generally limited by the neck diameter of the cathode ray tube. While such a limitation is not applicable to the magnetic lens system, magnetic lens systems consume additional electrical power and add additional weight (relative to the electrostatic focusing system).
U.S. Pat. No. 3,143,681 discloses the reduction of a spherical aberration in an electrostatic lens that is not limited by the neck diameter. In this patent, the neck diameter of the CRT is effectively enlarged by means of an electrostatic lens comprising a resistive structure having a coil pattern.
An electrostatic lens having a resistive structure comprising plural coil segments and plural intermediate segments that are alternately disposed is disclosed in Japanese Unexamined Patent Publication SHO 63-225464.
An article published at pages 134 to 136 of a magazine entitled OPTIK (Vol. 72, No. 4, 1986) describes a method for determining the length of the coil segments and intermediate segments in the direction of an electron beam trajectory. This article teaches that a minimum diameter of a beam spot formed on the screen is proportional to C0.sup.1/4, where C0 is equal to a minimum value (e.g., a minimum beam spot coefficient) of a beam spot coefficient C shown in equation (2), below. A spherical aberration coefficient Cs of the electrostatic lens used to calculate equation (2) is calculated from an integral of a z-axis, i.e., a direction of travel of an electron beam, based on equation (4), below. ##EQU1##
where:
M is equal to a magnification of the electrostatic lens; PA0 Vs is equal to a voltage on an emission side; PA0 V0 is equal to a voltage on an incidence side; PA0 Z0 is equal to a z-axis coordinate of an electro-optical object point produced by the electron beam; PA0 Zs is equal to the object point (i.e., the z-axis coordinate of the spot formed on the screen); PA0 r is equal to a radius of a paraxial path of the electron beam emitted at an angle of 1 radian from the object point; PA0 V is equal to a potential distribution on a central axis of a tubular substrate; and PA0 V', V", and V'" are first, second, and third derivatives of potential distribution V. PA0 C is equal to a beam spot coefficient; PA0 Vs is equal to a voltage on an emission side; PA0 V0 is equal to a voltage on an incidence side; PA0 Z0 is equal to a z-axis coordinate of an electro-optical object point produced by the electron beam; PA0 Zs is equal to the object point (i.e., the z-axis coordinate of the spot formed on the screen); PA0 r is equal to a radius of a paraxial path of the electron beam emitted at an angle of 1 radian from the object point; PA0 V is equal to a potential distribution on a central axis of a tubular substrate; PA0 V', V", and V'" are first, second, and third derivatives of potential distribution V; and PA0 r'(Zs) is a slope at an image point (Z=Zs) on a paraxial path of the electron beam emitted at the angle of 1 radian from the object point (Z=Z0). PA0 M equals a minimum magnification value; PA0 C0 equals a minimum value of the beam spot coefficient C; PA0 Cm equals the confirmed minimum value of the beam spot coefficient; PA0 parameter "a" equals a modification parameter value; and PA0 F equals the aberration-independent function. PA0 C is equal to a beam spot coefficient; PA0 M is equal to an electrostatic lens magnification; PA0 Vs is equal to a voltage on an emission side; PA0 V0 is equal to a voltage on an incidence side; PA0 Z0 is equal to a z-axis coordinate of an electro-optical object point produced by the electron beam; PA0 Zs is equal to the object point (i.e., the z-axis coordinate of the spot formed on the screen); PA0 r is equal to a radius of a paraxial path of the electron beam emitted at an angle of 1 radian from the object point; PA0 V is equal to a potential distribution on a central axis of a tubular substrate; PA0 V', V", and V'" are first, second, and third derivatives of potential distribution V; and PA0 r'(Zs) is a slope at an image point (Z=Zs) on a paraxial path of the electron beam emitted at the angle of 1 radian from the object point (Z=Z0). PA0 M0 equals a minimum magnification value; PA0 Cm equals the confirmed minimum value of the beam spot coefficient; PA0 a equals a modification parameter value; and PA0 F equals the aberration-independent function. PA0 Cm equals a minimized beam spot coefficient; PA0 a equals a modification parameter; PA0 M equals a magnification of the electrostatic lens; PA0 Cs equals a spherical aberration coefficient of the electrostatic lens; PA0 Vs equals an emission-side voltage; PA0 V0 equals an incidence-side voltage; and PA0 F equals an aberration-independent function. PA0 M0 equals a minimum value of the magnification M; and PA0 C0 equals a minimum beam spot coefficient value.
As disclosed in the OPTIK article, equations (2) and (4) are repeatedly calculated using a computer simulation model to determine a length of plural tubular film members of the CRT and a length of plural coil members of the CRT, until the beam spot coefficient C obtained by equation (2), above, is reduced to the minimum beam spot coefficient C0.
An electron gun constructed in accordance with the OPTIK article for forming a unipotential electrostatic lens in which the incidence-side voltage V0 is equal to an emissions-side voltage Vs is further disclosed in Japanese Unexamined Patent Publication HEI 7-73818.
However, when the specified voltage is applied to the resistive structure to drive the conventional CRT disclosed above, a sufficiently small beam spot (required for producing the high resolution (high definition) image) cannot be formed on the screen. That is, the empirical results obtained from the actual construction of a CRT in accordance with the above differs from the theoretical results produced by the computer simulation. Specifically, the actual diameter of the beam spot of a CRT produced with conventional manufacturing methods differs by approximately 10% to 20%, as compared to the results indicated by the computer simulations.
It is also known that the diameter of the beam spot varies over time as an electrical power supply to the electron gun is varied. This time-based fluctuation in the beam spot diameter results in instability in the brightness of the displayed image over time.